Ball valves are a well known type of valve. In a standard ball valve, a spherical ball is supported by a housing. The ball is able to rotate within the housing. The housing has inlet and outlet ports of a bore that is provided through the housing. The ball is positioned within the path of the bore through the housing. The ball has a straight cylindrical bore through it. When the valve is open, the bore through the ball is positioned relative to the bore through the housing such that a fluid is able to flow into the inlet port of the housing, through the bore of the ball and out of the outlet port of the housing. To close the valve the ball is rotated within the housing such that there is no fluid communication between the bore of the ball and the bore through the housing. For a standard ball valve, a rotation of the ball within the housing of 90 degrees changes the valve between the fully open and fully closed states. Within the ball valve, two seats in the housing provide the contact areas around the bore with the ball. When the valve is closed, it is necessary for the contact between the seats and the ball to provide a seal to ensure that substantially no fluid that flows into the inlet of the housing is able to flow to the outlet of the housing or leak in or out of the housing. When the valve is open, the fluid should flow through the bore from the inlet to the outlet, but this should be the only flow path for the fluid and no fluid should leak in or out of the housing.
Some applications of ball valves require a high standard of performance under demanding conditions. For example, some subsea applications in the oil and gas industry require seals that maintain a low leakage risk of fluid over a 40 year time period. The seal should function at extremely high and low temperatures with a wide range of hydraulic fluids, chemicals and well formation liquids that can comprise a mixture of oil, gas, water and sand.
In addition, the torque required to operate the valve should be maintained within strict limits. The opening and closing of subsea valves is normally performed by remote operated vehicles, ROVs, and the torque required to rotate the ball of the ball valve should therefore never exceed the maximum torque that can be applied by an ROV.
Known designs of ball valve have a high risk of failing to maintain low leakage requirements over a long time period in subsea applications. In order to maintain the seal between the seat and the ball valve, it is necessary for a positive pressure to be applied by each of the seats against the ball. It is known for the seat to be made of an polymeric material, such as the theroplastcs polytetrafluoroethylene (PTFE) and nitrile butadiene (NBR). The polymeric material is pressed into the ball to seal the contact. However, such polymeric seals are not suitable for long term use with certain types of fluid over a long time period or extremely high or low temperatures. In particular, dirty, or abrasive, fluids comprising sand are extremely detrimental to polymeric seals. Accordingly, polymeric seals that are in contact with well fluids have a high probability of failing in subsea applications and are not reliable.
There are known ball valves that have metal seats and a metal-to-metal seal is provided between the seat and the ball. These designs of ball valve provide a seat that is suitable for use with abrasive/dirty fluids, and at extreme temperatures, over long time periods. An example of a ball valve with metal seats is the 9800 Series by J Flow Controls, http://www.jflowcontrols.com/assets/downloads/Flanged-9800.pdf (viewed on 11 May 2015). A spring is provided behind the seats to force the seats against the ball and maintain a mechanical seal. A polymeric seal is still required between the seat, which is moveable by the spring, and the housing of the ball valve. Another problem is that the mechanical seal between the seat and the ball is not perfect and there is a small amount of fluid leakage through the seal. A secondary sealing system is therefore required. The use of polymeric materials and springs in the ball valve increases the likelihood of failure over long time periods and also complicates the design and manufacture of the valve.
Accordingly, there is a need to improve on known designs of ball valve, in particular for use in subsea applications.